Factor structure and relationship between core features of autism (social impairments, communication difficulties, and restricted, repetitive behaviours or interests (RRBIs)) were explored in 189 children from the Twins Early Development Study, diagnosed with autistic spectrum disorders (ASDs) using the Development and Wellbeing Assessment (DAWBA; Goodman et al. in J Child Psychol Psyc 41:645-655, 2000). A bottom-up approach (analysis 1) used principal component factor analysis of DAWBA items indicating five factors, the first three mapping on the triad. In analysis 2, applying top-down DSM-IV criteria, correlations between domains were modest, strongest between social and communication difficulties. Cross-twin cross-trait correlations suggested small shared genetic effects between RRBIs and other symptoms. These findings from a clinical sample of twins indicate a fractionation of social/communicative and RRBI symptoms in ASD.